Zircon compatible, ion exchangeable glass with high damage resistance

ABSTRACT

An ion exchangeable glass having a high degree of resistance to damage caused by abrasion, scratching, indentation, and the like. The glass comprises alumina, B 2 O 3 , and alkali metal oxides, and contains boron cations having three-fold coordination. The glass, when ion exchanged, has a Vickers crack initiation threshold of at least 10 kilogram force (kgf).

This application is a continuation of U.S. application Ser. No. 13/903,433 filed on May 28, 2013, which claims the benefit of priority under 35 U.S.C. §119 of U.S. Provisional Application Ser. No. 61/653,489, filed on May 31, 2012, and U.S. Provisional Application Ser. No. 61/748,981, filed on Jan. 4, 2013, the contents of all of the above being incorporated herein by reference in their entirety.

BACKGROUND

The disclosure relates to glasses that are capable of chemical strengthening by ion exchange and have intrinsic or “native” damage resistance. More particularly, the disclosure relates to such glasses that are strengthened by ion exchange and possess resistance to damage by abrasion, scratching, indentation, and other forms of sharp contact.

The ion exchange process provides a substantial improvement of glasses that are capable of being strengthened by this process to resist damage by sharp impact or indentation. To date, glasses containing network modifiers such as alkali and alkaline earth cations have been used. These cations form non-bridging oxygens (oxygens bonded to only one silicon atom), which reduce the resistance of the ion exchanged glass to damage introduced by abrasion, scratching, or the like.

SUMMARY

The present disclosure provides an ion exchangeable glass having a high degree of resistance to damage caused by abrasion, scratching, indentation, and the like. The glass comprises alumina, B₂O₃, and alkali metal oxides, and contains boron cations having three-fold coordination. The glass, when ion exchanged, has a Vickers crack initiation threshold of at least 10 kilogram force (kgf).

Accordingly, one aspect of the disclosure is to provide an ion exchangeable glass comprising at least about 50 mol % SiO₂; at least about 10 mol % R₂O, wherein R₂O comprises Na₂O; Al₂O₃, wherein Al₂O₃(mol %)<R₂O(mol %); and B₂O₃, and wherein B₂O₃(mol %)−(R₂O(mol %)−Al₂O₃(mol %))≧3 mol %.

A second aspect of the disclosure is to provide a glass comprising at least about 50 mol % SiO₂; at least about 10 mol % R₂O, wherein R₂O comprises Na₂O; Al₂O₃; and B₂O₃, wherein B₂O₃(mol %)−(R₂O(mol %)−Al₂O₃(mol %))≧3 mol %, and wherein the glass has a zircon breakdown temperature that is equal to the temperature at which the glass has a viscosity in a range from about 25 kPoise to about 40 kPoise.

A third aspect of the disclosure is to provide an ion exchanged glass having a Vickers crack initiation threshold of at least about 10 kgf. The glass comprises at least about 50 mol % SiO₂; at least about 10 mol % R₂O, wherein R₂O comprises Na₂O; Al₂O₃, wherein Al₂O₃(mol %)<R₂O(mol %); and B₂O₃, wherein B₂O₃(mol %)−(R₂O(mol %)−Al₂O₃(mol %))≧3 mol %.

A fourth aspect is to provide a glass comprising: at least about 50 mol % SiO₂; at least about 10 mol % R₂O, wherein R₂O comprises Na₂O; Al₂O₃; and at least 2.7 mol % B₂O₃ containing coordinated boron cations, wherein B₂O₃—(R₂O—Al₂O₃)≧3 mol %.

A fifth aspect is directed to a glass comprising: at least about 50 mol % SiO₂; at least about 10 mol % R₂O, wherein R₂O comprises Na₂O; Al₂O₃, wherein Al₂O₃(mol %)<R₂O(mol %); and 3-4.5 mol % B₂O₃, and wherein B₂O₃(mol %)−(R₂O(mol %)−Al₂O₃(mol %))≧3 mol %, and wherein the glass is ion exchangeable.

A sixth aspect is directed to a glass comprising: 66-74 mol % SiO₂; at least about 10 mol % R₂O, wherein R₂O comprises Na₂O; Al₂O₃, wherein Al₂O₃(mol %)<R₂O(mol %); and B₂O₃, wherein 4.5 mol %≧B₂O₃(mol %)−(R₂O(mol %)−Al₂O₃(mol %))≧3 mol %, and wherein the glass is ion exchangeable.

A seventh aspect is directed to a glass comprising 66-74 mol % SiO₂; at least about 10 mol % R₂O, wherein R₂O comprises Na₂O; Al₂O₃, wherein Al₂O₃(mol %)<R₂O(mol %); and 3-4.5 mol % B₂O₃, wherein B₂O₃(mol %)−(R₂O(mol %−Al₂O₃(mol %))≧3 mol %, wherein the glass comprises three-fold coordinated boron cations, and the glass is ion exchangeable.

These and other aspects, advantages, and salient features will become apparent from the following detailed description, the accompanying drawings, and the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plot of nuclear magnetic resonance (NMR) spectra for samples of the glasses described herein.

DETAILED DESCRIPTION

In the following description, like reference characters designate like or corresponding parts throughout the several views shown in the figures. It is also understood that, unless otherwise specified, terms such as “top,” “bottom,” “outward,” “inward,” and the like are words of convenience and are not to be construed as limiting terms. In addition, whenever a group is described as comprising at least one of a group of elements and combinations thereof, it is understood that the group may comprise, consist essentially of, or consist of any number of those elements recited, either individually or in combination with each other. Similarly, whenever a group is described as consisting of at least one of a group of elements or combinations thereof, it is understood that the group may consist of any number of those elements recited, either individually or in combination with each other. Unless otherwise specified, a range of values, when recited, includes both the upper and lower limits of the range as well as any ranges therebetween. As used herein, the indefinite articles “a,” “an,” and the corresponding definite article “the” mean “at least one” or “one or more,” unless otherwise specified. It also is understood that the various features disclosed in the specification and the drawings can be used in any and all combinations.

As used herein, the terms “glass” and “glasses” includes both glasses and glass ceramics. The terms “glass article” and “glass articles” are used in their broadest sense to include any object made wholly or partly of glass and/or glass ceramic.

It is noted that the terms “substantially” and “about” may be utilized herein to represent the inherent degree of uncertainty that may be attributed to any quantitative comparison, value, measurement, or other representation. These terms are also utilized herein to represent the degree by which a quantitative representation may vary from a stated reference without resulting in a change in the basic function of the subject matter at issue.

Unless otherwise specified, all compositions and concentrations described herein are expressed in mole percent (mol %). It is understood that the compositional ranges described and claimed herein represent the bulk composition of the glass as determined by those means skilled in the art, and are applicable to both the unstrengthened and strengthened glasses described herein.

Referring to the drawings in general and to FIG. 1 in particular, it will be understood that the illustrations are for the purpose of describing particular embodiments and are not intended to limit the disclosure or appended claims thereto. The drawings are not necessarily to scale, and certain features and certain views of the drawings may be shown exaggerated in scale or in schematic in the interest of clarity and conciseness.

Accordingly, a glass having intrinsic damage resistance is provided. In one aspect, the glass, which may be strengthened so as to be resistant to damage via sharp impact, comprises at least about 50 mol % SiO₂; at least about 10 mol % of at least one alkali metal oxide R₂O, wherein R₂O includes Na₂O and, optionally, other alkali metal oxides (e.g., Li₂O, K₂O, Ce₂O, Rb₂O); alumina (Al₂O₃), wherein the amount of Al₂O₃, expressed in mol %, is less than the total amount of the alkali metal oxides present in the glass (i.e., Al₂O₃(mol %)<R₂O(mol %)); and boron oxide (B₂O₃), wherein B₂O₃(mol %)−(R₂O(mol %)−Al₂O₃(mol %))≧3 mol %.

In another aspect, the glass comprises at least about 50 mol % SiO₂; Al₂O₃; B₂O₃; and at least about 10 mol % R₂O, wherein R₂O comprises Na₂O; wherein B₂O₃(mol %)−(R₂O(mol %)−Al₂O₃(mol %))≧3 mol %, and wherein the glass has a zircon breakdown temperature (i.e., the temperature at which zircon breaks down to form zirconia and silica) equal to the temperature at which the viscosity of the glass is in a range from about 25 kPoise to about 40 kPoise.

In some aspects, the glasses described herein are ion exchangeable; i.e., cations—typically monovalent alkali metal cations—which are present in these glasses are replaced with larger cations—typically monovalent alkali metal cations, although other cations such as Ag⁺ or Tl⁺—having the same valence or oxidation state. The replacement of smaller cations with larger cations creates a surface layer that is under compression, or compressive stress CS. This layer extends from the surface into the interior or bulk of the glass to a depth of layer DOL. The compressive stress in the surface layers of the glass are balanced by a tensile stress, or central tension CT, in the interior or inner region of the glass. Compressive stress and depth of layer are measured using those means known in the art. Such means include, but are not limited to measurement of surface stress (FSM) using commercially available instruments such as the FSM-6000, manufactured by Luceo Co., Ltd. (Tokyo, Japan), or the like. Methods of measuring compressive stress and depth of layer are described in ASTM 1422C-99, entitled “Standard Specification for Chemically Strengthened Flat Glass,” and ASTM 1279.19779 “Standard Test Method for Non-Destructive Photoelastic Measurement of Edge and Surface Stresses in Annealed, Heat-Strengthened, and Fully-Tempered Flat Glass,” the contents of which are incorporated herein by reference in their entirety. Surface stress measurements rely upon the accurate measurement of the stress optical coefficient (SOC), which is related to the stress-induced birefringence of the glass. SOC in turn is measured by those methods that are known in the art, such as fiber and four point bend method, both of which are described in ASTM standard C770-98 (2008), entitled “Standard Test Method for Measurement of Glass Stress-Optical Coefficient,” the contents of which are incorporated herein by reference in their entirety, and a bulk cylinder method.

In other aspects, the glasses described herein are ion exchanged and have at least one surface having a layer that is under compression to a depth of layer and an inner region that is under a tensile stress, as described hereinabove. In one aspect, the ion exchanged glass comprises at least about 50 mol % SiO₂; at least about 10 mol % R₂O, wherein R₂O comprises Na₂O; Al₂O₃; and B₂O₃, and wherein B₂O₃—(R₂O—Al₂O₃)≧3 mol %. The ion exchanged glass has a Vickers crack initiation threshold of at least about 10 kilogram force (kgf).

When present in boroaluminosilicate glasses, alkali metals act as charge-balancing cations for tetrahedrally coordinated aluminum and boron. Species similar to RAlO₂ or RBO₂, where R is an alkali metal and aluminum are Al³⁺ and B³⁺, for example, are four-fold coordinated by oxygen. The four oxygen atoms surrounding aluminum or boron are presumed to bond to silicon atoms. Each oxygen therefore obtains one electron from the silicon atoms and, in effect, ¾ of an electron from the tetrahedrally coordinated Al³⁺ or B³⁺. An alkali metal donates an electron to this configuration, thus allowing all four oxygens in the tetrahedron to obtain a full electron shell. The large alkaline earth elements Ca, Sr, and Ba are known to fill this role for aluminum. Thus, in a boroaluminosilicate glass in which R₂O(mol %)+CaO(mol %)+SrO+BaO(mol %)−Al₂O₃(mol %)−B₂O₃(mol %)=0(mol %) or, alternatively, (Al₂O₃(mol %)+B₂O₃(mol %))/(R₂O(mol %)+CaO(mol %)+SrO(mol %)+BaO(mol %))=1, there are exactly as many electrons donated from alkalis and alkaline earths as there are tetrahedrally coordinated aluminum and boron atoms in need of charge balance. When R₂O(mol %)+CaO(mol %)+SrO(mol %)+BaO(mol %)−Al₂O₃(mol %)−B₂O₃(mol %)<0(mol %), there are more aluminum and boron atoms in need of charge balance than electrons from charge-balancing alkali or alkaline earth cations. Such glasses generally have high intrinsic or “native” damage resistance, which is considerably enhanced after ion exchange.

In these glasses, aluminum is nearly always four-fold coordinated by oxygen if enough charge-balancing cations are available. Low-valence cations perform better in a charge-balancing role than small, higher valence cations. This has been attributed to the extent to which the charge-balancing cation competes with aluminum for electron density. Large, low-valence cations are ineffective in this competition and thus leave electron density largely within the AlO₄ tetrahedron. Boron is mostly four-fold coordinated in fully annealed alkali borosilicate glasses in which the amount of alkali oxide present in the glass is less than or equal to than the concentration Al₂O₃, as the alkali modifiers are consumed or taken up in bonds with aluminum. In fully annealed alkali borosilicate glasses in which the amount of alkali oxide present in the glass is greater than the combined concentration of B₂O₃ and Al₂O₃, boron is mostly four-fold coordinated and the excess alkali modifiers are taken up in non-bridging oxygen (NBO) bonds. In fully annealed boroaluminosilicate glasses in which B₂O₃≧alkali modifiers>Al₂O₃, however, boron assumes a mix of three and four-fold coordination, even in charge-balanced compositions. When fast-cooled from above the anneal or strain point of the glass, the formation of three-fold coordinated boron is entropically preferred. Thus, the proportion of three-fold coordinated boron may be enhanced through forming processes such as down-draw (e.g., slot- or fusion-draw) processes, or by heating the glass to a temperature above tits anneal or strain point, followed by fast-cooling (e.g., at rates of at least about 4° C./s). In some embodiments, the glass is fast-cooled from a temperature that is above the anneal or strain point of the glass. To a reasonable approximation, charge-balancing cations first stabilize aluminum in four-fold coordination, and the remaining cations stabilize some fraction of boron in four-fold coordination, where the fraction may be as high as 1. The glasses described herein comprise at least 2.7 mol % and, in some embodiments, from about 2.7 mol % to about 4.5 mol % B₂O₃, in which the boron cations are predominantly three-fold coordinated. In some embodiments, at least about 50% of the boron cations comprising the glasses described herein are three-fold coordinated. In other embodiments, at least about 80% of the boron in these glasses is three-fold coordinated and, in still other embodiments, at least 90% of the boron cations in these glasses are three-fold coordinated. The proportion of three fold boron can be determined using ¹¹B nuclear magnetic resonance (NMR). FIG. 1 shows NMR spectra for samples of the glasses described herein having the composition 67.45 mol % SiO₂, 12.69 mol % Al₂O₃, 3.67 mol % B₂O₃, 13.67 mol % Na₂O, 2.36 mol % MgO, and 0.09 mol % SnO₂. The amount of four-fold coordinated boron (N₄ in FIG. 1) ranges from 8.9% to 9.5%. Thus, about 90% of the boron present in each of the samples shown in FIG. 1 is three-fold coordinated.

When R₂O(mol %)+MO(mol %)<Al₂O₃(mol %), where MO represents divalent metal oxides (e.g., MgO, ZnO, CaO, BaO, SrO, etc.), there are not enough monovalent or divalent cations to stabilize aluminum in four-fold coordination, and boron is therefore almost entirely in three-fold coordination in these glasses. In some embodiments, the glasses described herein comprise about 3 mol % to about 4.5 mol % of B₂O₃ which contains boron cations that are surrounded by three anions; i.e., the boron cations are three-fold coordinated and, in some embodiments, B₂O₃(mol %)−(R₂O(mol %)−Al₂O₃(mol %))≦4.5 mol %. Three-fold coordinated boron is desirable because it densifies more readily when compressed, thus providing intrinsic or native damage resistance to the glass. Glasses with compositions in which R₂O(mol %)+MO(mol %)−Al₂O₃(mol %)<0(mol %), however, generally have very high melting temperatures unless they also have very high B₂O₃ contents, e.g., greater than about 10 mol %. Such high amounts of B₂O₃ have a deleterious effect on both the surface compressive stress and rate of ion exchange, as evidenced by low depths of layer obtained in an ion exchange of fixed duration/time period. Since high CS (e.g., greater than about 650 MPa) and significant depth of layer (DOL) (e.g., greater than 30 μm) are also required to obtain good damage resistance, the additional benefits of high intrinsic damage resistance may be offset by poor ion exchange characteristics. In order to achieve optimal balance of attributes after ion exchange, it is therefore desirable to keep boron concentrations sufficiently low. Accordingly, in some embodiments, the glasses described herein comprise less than 10 mol % B₂O₃. Such glasses are very difficult to melt to a level of quality (e.g., defect concentration) acceptable for mass manufacturing. Depending on the application for the glass article, a marginally acceptable defect level in a continuous process is about 1 inclusion (e.g., gas bubbles, particles, or “seeds”) that are greater than about 50 μm in size (e.g., diameter, for approximately spherical particles, or major axis for elongated particles) per pound (lb) of glass.

In some embodiments, the glasses described herein comprise at least about 50 mol % SiO₂, from about 9 mol % to about 22 mol % Al₂O₃; from about 3 mol % to about 10 mol % B₂O₃; from about 10 mol % to about 20 mol % Na₂O; from 0 mol % to about 5 mol % K₂O; at least about 0.1 mol % MgO and/or ZnO, wherein 0 mol %≦MgO(mol %)+ZnO(mol %)≦6 mol %; and, optionally, at least one of CaO, BaO, and SrO, wherein 0 mol %≦CaO(mol %)+SrO(mol %)+BaO(mol %)≦2 mol %. In some embodiments, the glasses described herein comprise from about 66 mol % to about 74 mol % SiO₂.

In some embodiments, the glasses described herein comprise at least about 0.1 mol % of at least one of MgO and ZnO. In certain embodiments, the glass may comprise up to about 6 mol % MgO and/or ZnO. Magnesium and zinc behave differently from the other divalent cations. While magnesium and zinc may charge-balance aluminum to some extent, they appear to have little or no role in stabilizing boron in four-fold coordination in the glasses described herein. Thus, replacing an alkali oxide with magnesium oxide in a boroaluminosilicate glass forces boron atoms out of four-fold coordination and into three-fold coordination, resulting in higher indentation crack initiation threshold load in ion-exchanged glass.

In addition to the oxides listed above, the glasses described herein may further comprise chemical fining agents including, but not limited to, halogens or halides (compounds containing F, Cl, Br and I) and at least one of the oxides As₂O₃, Sb₂O₃, CeO₂, and SnO₂. These fining agents, when present, are generally batched into the raw material at a level of about 0.5 mol % or less, and have minor impact on both the rate of ion exchange and the compressive stress that are ultimately obtained. In addition, other oxides may be added at low concentrations with little or no impact on the ion exchange characteristics of these glasses. Examples of such oxides include ZrO₂, which is a common contaminant introduced by zirconia refractories in melters; TiO₂, which is a common tramp component of natural silica sources; Fe₂O₃, which is an ubiquitous tramp oxide in all but the purest chemical reagents; and other transition metal oxides that may be used to introduce color. These transition metal oxides include V₂O₅, Cr₂O₃, Co₃O₄, Fe₂O₃, MnO₂, NiO, CuO, Cu₂O, and the like. Concentrations of these oxides, when present in the glass, are kept at levels of less than or equal to about 2 mol %. Compared to calcium, larger alkaline earths such as Sr and Ba lead to lower diffusivity and thus significantly lower depth of the compressive layer. Because Sr and Ba are costly reagents compared to most of the other oxides in the glasses described herein and inhibit or retard ion exchange. The concentrations of SrO and BaO, when present in the glass, are therefore kept at levels of no more than about 0.5 mol %. Rubidium and cesium oxides are too large to ion exchange at an appreciable rate, are costly, and contribute to high liquidus temperatures at elevated concentrations. The concentrations of these oxides are therefore kept below 0.5 mol %. Lithium oxide is to be generally avoided, as it contributes to “poisoning” of potassium nitrate salt ion exchange baths, resulting in lower compressive stress than if the same glass were ion exchanged in a lithium-free salt bath. In addition, compared to a sodium-for-potassium exchange, the presence of lithium leads to significantly reduced diffusivity when exchanging for potassium. When present in the glass, the lithium oxide concentration should therefore be kept below about 0.5 mol %, and, in some embodiments, below about 0.1 mol %. In some embodiments, the glasses described herein are substantially free of lithium. In some embodiments, the glasses described herein are substantially free of phosphorus.

As used herein, the term “zircon breakdown temperature” or “T^(breakdown)” refers to the temperature at which zircon—which is commonly used as a refractory material in glass processing and manufacture—breaks down to form zirconia and silica. In isoviscous processes such as fusion, the highest temperature experienced by the glass corresponds to a particular viscosity of the glass. For example, “T^(35 kP)” refers to the temperature at which the glass has a viscosity of 35 kilopoise (kP). The difference between the breakdown temperature and the temperature corresponding to 35,000 poise viscosity is defined as the breakdown margin T^(margin), where: T^(margin)=T^(breakdown)−T^(35 kp). When the breakdown margin T^(margin) is negative, zircon will breakdown to form zirconia defects at some location on the fusion isopipe. When T^(margin) is zero, it is still possible that temperature excursions could cause zircon breakdown to occur. It is therefore desirable not only to make the breakdown margin positive, but to maximize T^(margin) as much as possible while being consistent with all the other attributes that must be maintained in the final glass product. In some embodiments, the glasses described herein have a zircon breakdown temperature that is equal to the temperature at which the viscosity of the glass is in a range from about 25 kPoise to about 40 kPoise, in some embodiments, in a range from about 30 kPoise to about 40 kPoise, and, in a particular embodiment, from 30 kPoise to about 35 kPoise. Zircon breakdown temperatures T^(breakdown) measured for various samples of the glasses described herein are listed in Table 3.

As previously mentioned, zircon will breakdown to form zirconia and silica at some location on processing hardware—such as, for example, a fusion isopipe—when the breakdown margin T^(margin) is negative. This often results in the inclusion of defects, bubbles, zirconia particles, or the like in the formed glass. The presence of such inclusions in the glass is typically detected by those means know in the art such as, for example, optical microscopy. The glasses described herein have positive zircon breakdown margins and, in some embodiments, have less than 0.1 inclusions per pound of glass, wherein the inclusions are at least about 50 μm in size (i.e., diameter, for approximately spherical particles, or major axis for elongated particles). In some embodiments, the glasses described herein contain less than 0.01 inclusions per pound of glass, wherein the inclusions are at least about 50 μm in size.

In those embodiments in which the glass is strengthened by ion exchange, the glass may, in some embodiments, be ion exchanged to create a surface layer that is under a compressive stress of at least about 600 megapascals (MPa) and, in other embodiments, at least about 800 MPa. The compressive surface layer has a depth of layer of at least about 30 microns (μm). The ion exchanged glasses described herein also possess a degree of native or intrinsic damage resistance (IDR), which may be characterized by a Vickers crack initiation threshold of greater than or equal to 10 kilogram force (kgf). In some embodiments, the ion exchanged glass has a Vickers crack initiation threshold in a range from about 20 kgf to about 30 kgf. In other embodiments, the ion exchanged glass has a Vickers crack initiation threshold in a range from about 30 kgf to about 35 kgf. The Vickers crack initiation threshold measurements described herein are performed by applying and then removing an indentation load to the glass surface at a rate of 0.2 mm/min. The maximum indentation load is held for 10 seconds. The crack initiation threshold is defined at the indentation load at which 50% of 10 indents exhibit any number of radial/median cracks emanating from the corners of the indent impression. The maximum load is increased until the threshold is met for a given glass composition. All indentation measurements are performed at room temperature in 50% relative humidity.

When ion exchanged, the glasses described herein have a compressive stress that is sufficiently high to provide outstanding damage resistance against impact of various kinds. The glasses described herein may be ion exchanged at a rate that facilitates large scale manufacturing. While it is advantageous to provide high compressive stress by annealing the glass prior to ion exchange, a high compressive stress may also be obtained by instead cooling the glass rapidly from a high temperature (e.g., above the strain point of the glass). Such rapid cooling may occur, for example, in down drawn processes such as fusion or slot draw processes. The combination of compressive stress, depth of layer, and intrinsic damage resistance of the ion exchanged glasses described herein provides excellent resistance to visible or strength-limiting damage introduced via sharp contact and scratching.

Physical properties of an exemplary sample of the glasses described herein, labeled Glass 3 and having the composition 67.45 mol % SiO₂, 12.69 mol % Al₂O₃, 3.67 mol % B₂O₃, 13.67 mol % Na₂O, 0.02 mol % K₂O, 2.36 mol % MgO, 0.03 mol % CaO, and 0.09 mol % SnO₂, are compared to those of glasses described in U.S. patent application Ser. No. 13/533,298, filed on Jul. 1, 2011, by Matthew J. Dejneka et al., entitled “Ion Exchangeable Glass With High Compressive Stress,” which claims priority from U.S. Provisional Patent Application No. 61/503,734, filed on Jul. 1, 2011, by Matthew J. Dejneka et al., and having the same title; and U.S. Provisional Patent Application No. 61/653,485, filed on May 31, 2012, by Matthew J. Dejneka et al., entitled “Ion Exchangeable Glass With High Damage Resistance;” labeled Glasses 1 and 2, respectively, in Table 1. Glass 1, having the composition 68.84 mol % SiO₂, 10.63 mol % Al₂O₃, 0 mol % B₂O₃, 14.86 mol % Na₂O, 0.02 mol % K₂O, 5.43 mol % MgO, 0.04 mol % CaO, and 0.17 mol % SnO₂, is compatible with zircon, but has a low indentation threshold, whereas Glass 2, having the composition 64.65 mol % SiO₂, 13.93 mol % Al₂O₃, 5.11 mol % B₂O₃, 13.75 mol % Na₂O, 0 mol % K₂O, 2.38 mol % MgO, 0.14 mol % CaO, and 0.08 mol % SnO₂, has a high indentation threshold but is incompatible with zircon. Glass 3 possesses a combination of high indentation threshold and compatibility with zircon, as described hereinabove.

TABLE 1 Physical properties of glasses. Glass 1 2 3 Anneal Point (° C.) 650 629 644 Strain Point (° C.) 601 576 589 Softening Point (° C.) 891.7 900 922.4 Density (g/cm³) 2.432 2.39 2.403 Poisson's Ratio 0.205 0.216 0.213 Shear Modulus (Mpsi) 4.287 4.051 4.142 Young's Modulus (Mpsi) 10.336 9.851 10.046 Liquidus Temperature (° C.) 1020 1000 1005 Liquidus Viscosity (kPoise) 1000 1850 2210 Primary Devit Phase Forsterite Nepheline Forsterite Zircon Breakdown 1200 1183 1230 Temperature (° C.) Zircon Breakdown 30.4 71.4 33.4 Viscosity (kPoise) 200 Poise Temperature (° C.) 1665 1679 1757 35 kPoise Temperature (° C.): 1190 1226 1227 Refractive Index 1.50030 1.49844 1.49836 SOC (nm/MPa/cm) 29.64 32.78 31.94 Vickers Indentation 7 >30 20-30 Threshold (kgf)

Additional non-limiting examples of the compositions of the glasses described herein and their properties are listed in Tables 2 and 3, respectively. The examples listed satisfy the requirement of having intrinsic damage resistance, as characterized by high indentation threshold, and compatibility with zircon, as characterized by low zircon breakdown viscosity.

TABLE 2 Examples of compositions of the glasses described herein. Analyzed Example (mol %) 1 2 3 4 5 6 7 SiO₂ 67.26 67.47 67.37 67.43 67.22 67.12 67.29 Al₂O₃ 12.05 12.08 12.07 12.03 12.03 12.03 12.05 B₂O₃ 2.58 2.56 2.54 2.61 2.61 2.64 2.64 Na₂O 14.14 13.08 14.10 13.10 14.20 13.33 13.20 K₂O 0.01 0.96 0.01 0.96 0.03 0.94 0.96 MgO 3.80 3.69 3.34 3.27 3.34 3.36 2.82 CaO 0.05 0.04 0.48 0.49 0.06 0.05 0.48 ZnO 0.00 0.00 0.00 0.00 0.41 0.42 0.45 SnO₂ 0.09 0.09 0.09 0.09 0.09 0.09 0.09 ZrO₂ 0.01 0.01 0.01 0.01 0.01 0.01 0.01 Fe₂O₃ 0.01 0.01 0.01 0.01 0.01 0.01 0.01 Total 100 100 100 100 100 100 100 (mol %) Analyzed Example (mol %) 8 9 10 11 12 13 14 SiO₂ 67.25 66.32 66.32 66.22 66.26 67.28 67.29 Al₂O₃ 12.04 12.73 12.76 12.72 12.74 12.04 12.03 B₂O₃ 2.63 3.53 3.64 3.62 3.63 3.41 3.44 Na₂O 13.30 13.93 12.89 13.07 13.06 13.87 12.93 K₂O 0.96 0.03 0.95 0.96 0.97 0.01 0.94 Li₂O MgO 2.76 3.31 2.84 2.85 2.32 2.79 2.77 CaO 0.05 0.05 0.48 0.05 0.47 0.49 0.49 ZnO 0.89 0.00 0.00 0.40 0.45 0.00 0.00 SnO₂ 0.09 0.09 0.09 0.09 0.09 0.09 0.09 ZrO₂ 0.01 0.01 0.02 0.01 0.01 0.01 0.01 Fe₂O₃ 0.01 0.01 0.01 0.01 0.01 0.01 0.01 P₂O₅ 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Total 100 100 100 100 100 100 100 (mol %) Analyzed Example (mol %) 15 16 17 18 19 20 21 SiO₂ 67.18 66.27 66.33 66.16 67.23 67.61 66.82 Al₂O₃ 12.00 12.74 12.73 12.73 12.72 12.24 12.59 B₂O₃ 3.39 3.54 3.53 3.58 3.63 3.64 3.51 Na₂O 14.10 14.11 14.12 14.19 13.91 13.96 14.47 K₂O 0.04 0.01 0.01 0.01 0.01 0.04 0.01 MgO 1.82 2.27 1.79 1.84 2.34 2.35 2.45 CaO 0.49 0.05 0.04 0.48 0.05 0.06 0.05 ZnO 0.88 0.90 1.33 0.91 0.00 0.00 0.00 SnO₂ 0.09 0.09 0.09 0.09 0.09 0.09 0.09 ZrO₂ 0.01 0.01 0.01 0.01 0.01 0.01 0.01 Fe₂O₃ 0.01 0.01 0.01 0.01 0.01 0.01 0.01 Total 100 100 100 100 100 100 100 (mol %) Analyzed Example (mol %) 22 23 24 25 26 27 28 SiO₂ 66.59 67.05 66.38 66.98 67.05 67.09 67.23 Al₂O₃ 12.41 12.16 12.71 12.69 12.56 12.67 12.67 B₂O₃ 3.42 2.91 3.56 3.98 3.99 3.62 4.10 Na₂O 13.40 13.34 14.19 13.91 13.55 14.16 13.97 K₂O 0.66 0.85 0.01 0.01 0.01 0.01 0.03 MgO 3.01 2.88 1.79 2.21 2.05 2.24 1.83 CaO 0.12 0.06 0.04 0.03 0.03 0.03 0.06 ZnO 0.28 0.64 1.19 0.06 0.65 0.06 0.00 SnO₂ 0.09 0.09 0.09 0.09 0.09 0.09 0.09 ZrO₂ 0.01 0.01 0.03 0.01 0.01 0.01 0.01 Fe₂O₃ 0.01 0.01 0.01 0.01 0.01 0.01 0.01 Total 100 100 100 100 100 100 100 (mol %) Analyzed Example (mol %) 29 30 31 32 33 34 35 SiO₂ 67.31 67.32 66.96 67.43 67.09 67.45 67.11 Al₂O₃ 12.54 12.65 12.63 12.56 12.66 12.46 12.57 B₂O₃ 4.25 3.76 3.96 3.93 4.15 4.07 4.12 Na₂O 13.62 13.76 13.84 13.54 13.64 13.50 13.64 K₂O 0.01 0.01 0.01 0.01 0.01 0.01 0.01 MgO 2.11 2.37 2.47 2.41 2.33 2.38 2.42 CaO 0.04 0.04 0.04 0.03 0.04 0.03 0.04 ZnO 0.00 0.00 0.00 0.00 0.00 0.00 0.00 SnO₂ 0.09 0.07 0.08 0.07 0.07 0.08 0.08 ZrO₂ 0.01 0.01 0.01 0.01 0.01 0.01 0.01 Fe₂O₃ 0.01 0.01 0.01 0.01 0.01 0.01 0.01 Total 100 100 100 100 100 100 100 (mol %)

TABLE 3 Properties of glasses listed in Table 2. Example 1 2 3 4 5 6 Anneal Point (° C.): 632 632 632 624 633 629 Strain Point (° C.): 579 580 581 573 581 577 Softening Point (° C.): 890.3 895.7 888.6 884 891.5 888.4 Density (g/cm³): 2.418 2.417 2.421 2.422 2.424 2.425 CTE (×10⁻⁷/° C.): 77.1 79.3 77.3 79.3 77.3 80.3 Poisson's Ratio: 0.216 0.202 0.211 0.21 0.202 0.214 Shear Modulus (Mpsi): 4.216 4.232 4.234 4.241 4.228 4.221 Young's Modulus 10.25 10.174 10.252 10.265 10.16 10.247 (Mpsi): 72-hr Pt Liquidus Temp 1060 1070 1010 1000 1030 1040 (° C.): Liquidus Viscosity 247 345 739 1458 810 810 (kPoise): Primary Devit Phase: Unknown Unknown Unknown Unknown Unknown Unknown 200-Poise Temp (° C.): 1744.5 1715.2 1733.2 1724.3 1727.5 1736.6 35-kPoise Temp (° C.): 1215.2 1210.8 1196.8 1199.6 1206.5 1213.5 Fictivation Temp (° C.): 721.8 715.8 714.3 704.5 717.2 717.1 Refractive Index: 1.50000 1.49992 1.50068 1.50104 1.50033 1.50051 SOC (nm/MPa/cm): 30.88 30.75 30.86 30.74 31.22 31.01 CS at 50 μm 989.6 954.8 979.6 938.6 968.8 941.4 DOL/410° C./1 mm/ Annealed/Refined (MPa): Time to 50 μm DOL at 18.5 13.8 20.4 15 18.3 13.1 410° C./Annealed/Refined (h): Predicted Indentation 15 16 14 14 16 15 Threshold at 410° C./8 h/ Fictivated (kgf): Zircon Breakdown 1210 1204 1207 1199 1206 1193 Temperature (° C.): Zircon Breakdown 38 39 30 35 36 48 Viscosity (kPoise): Example 7 8 9 10 11 12 Anneal Point (° C.): 625 629 629 622 623 620 Strain Point (° C.): 573 576 577 570 571 568 Softening Point (° C.): 883.9 889.8 890.6 885.6 889.8 883.1 Density (g/cm³): 2.428 2.422 2.411 2.414 2.418 2.421 CTE (×10⁻⁷/° C.): 79.3 79.9 76.5 78.8 78.8 78.4 Poisson's Ratio: 0.221 0.209 0.213 0.209 0.212 0.215 Shear Modulus (Mpsi): 4.241 4.231 4.164 4.197 4.189 4.193 Young's Modulus 10.36 10.228 10.106 10.148 10.153 10.192 (Mpsi): 72-hr Pt Liquidus Temp 970 995 1065 1025 1040 960 (° C.): Liquidus Viscosity 2799 1401 287 939 805 4180 (kPoise): Primary Devit Phase: Albite Unknown Unknown Unknown Unknown Unknown 200-Poise Temp (° C.): 1721.0 1729.6 1716.9 1718.2 1722.0 1714.0 35-kPoise Temp (° C.): 1205.1 1205.4 1212.1 1209.7 1213.1 1204.1 Fictivation Temp (° C.): 1.50124 1.50102 1.49967 1.50039 1.50016 1.50090 Refractive Index: 30.67 31.18 31.5 31.13 31.54 31.53 SOC (nm/MPa/cm): 957.4 945.5 968.3 942.5 927 927.6 CS at 50 μm 14.4 13.6 18.3 15.3 13.7 15.7 DOL/410° C./1 mm/ Annealed/Refined (MPa): Time to 50 μm DOL at 11 15 20 18 18 17 410° C./Annealed/Refined (h): Zircon Breakdown 1192 1192 1202 1190 1187 1182 Temperature (° C.): Zircon Breakdown 43 43 41 48 52 49 Viscosity (kPoise): Example 13 14 15 16 17 18 Anneal Point (° C.): 619 617 617 627 626 622 Strain Point (° C.): 568 567 567 574 573 571 Softening Point (° C.): 877.5 876.6 871.5 890.5 889.1 881.2 Density (g/cm³): 2.415 2.415 2.429 2.425 2.432 2.43 CTE (×10⁻⁷/° C.): 76.3 78.8 76.1 76.1 76.1 76.2 Poisson's Ratio: 0.223 0.203 0.216 0.216 0.205 Shear Modulus (Mpsi): 4.221 4.226 4.158 4.145 4.197 Young's Modulus 10.324 10.171 10.117 10.083 10.119 (Mpsi): 72-hr Pt Liquidus Temp 960 950 970 970 950 965 (° C.): Liquidus Viscosity 2975 2742 3595 2933 3783 2470 (kPoise): Primary Devit Phase: Albite Albite Albite Unknown Albite Albite 200-Poise Temp (° C.): 1692.6 1658.8 1815.2 1714.7 1725.1 1717.9 35-kPoise Temp (° C.): 1179.0 1165.6 1226.2 1208.5 1207.9 1199.5 Refractive Index: 31.07 31.11 32.52 32.02 32.19 31.79 SOC (nm/MPa/cm): 933.5 908.4 969.6 976.2 963.9 CS at 50 μm 21.8 16.4 19.7 19.5 21.3 DOL/410° C./1 mm/ Annealed/Refined (MPa): Time to 50 μm DOL at 13 16 20 21 19 410° C./Annealed/Refined (h): Zircon Breakdown 1218 1208 1205 1193 1191 1184 Temperature (° C.): Zircon Breakdown 19 18 47 45 46 44 Viscosity (kPoise): Example 19 20 21 22 23 24 Anneal Point (° C.): 629 621 621 629 629 626 Strain Point (° C.): 576 569 570 577 576 574 Softening Point (° C.): 898.9 884.3 877.3 893.2 896.2 890.2 Density (g/cm³): 2.404 2.406 2.412 2.42 2.426 2.429 CTE (×10⁻⁷/° C.): 76.4 76.8 78.1 78.2 78.6 76.1 Poisson's Ratio: 0.208 0.209 0.215 0.212 0.222 0.219 Shear Modulus (Mpsi): 4.141 4.161 4.152 4.222 4.222 4.158 Young's Modulus 10 10.058 10.09 10.232 10.315 10.138 (Mpsi): 72-hr Pt Liquidus Temp 980 965 960 1030 1020 955 (° C.): Liquidus Viscosity 980 950 960 1020 1020 945 (kPoise): Primary Devit Phase: 3592 3855 3649 904 1358 5875 200-Poise Temp (° C.): Unknown Albite Albite Unknown Unknown Albite 35-kPoise Temp (° C.): 1793.2 1760.7 1749.9 1715.3 1723.8 1705.4 Fictivation Temp (° C.): 1224.1 1209.5 1196.8 1206.8 1215.8 1205.7 Refractive Index: 1.49821 1.49833 1.49955 1.50044 1.50084 1.50117 SOC (nm/MPa/cm): 31.99 31.55 31.44 31.49 31.25 32 CS at 50 μm 944.9 920.7 944.7 1018.3 DOL/410° C./1 mm/ Annealed/Refined (MPa): Time to 50 μm DOL at 15.4 16.3 17.6 16.3 410° C./Annealed/Refined (h): Predicted Indentation 23 21 21 15 13 20 Threshold at 410° C./8 h/ Fictivated (kgf): Zircon Breakdown 1216 1224 1199 1205 1195 1220 Temperature (° C.): Zircon Breakdown 39 28 34 36 49 28 Viscosity (kPoise): Example 25 26 27 28 29 30 Anneal Point (° C.): 628 629 630 635 629 640 Strain Point (° C.): 576 576 577 582 576 586 Softening Point (° C.): 894.5 898 896.4 905.8 898.9 904.7 Density (g/cm³): 2.405 2.412 2.409 2.405 2.402 2.402 CTE (×10⁻⁷/° C.): 76.4 74.4 76.5 75.9 75 75.3 Poisson's Ratio: 0.225 0.219 0.206 0.207 0.223 0.214 Shear Modulus (Mpsi): 4.152 4.135 4.165 4.099 4.212 4.126 Young's Modulus 10.173 10.079 10.048 9.893 10.299 10.02 (Mpsi): 72-hr Pt Liquidus Temp 980 990 965 960 970 1000 (° C.): Liquidus Viscosity 970 970 950 945 950 995 (kPoise): Primary Devit Phase: 3223 2744 4960 5696 4346 2738 200-Poise Temp (° C.): Unknown Unknown Albite Albite Forsterite Unknown 35-kPoise Temp (° C.): 1725.2 1739.8 1729.2 1741.3 1744.1 1753.8 Fictivation Temp (° C.): 1213.2 1216.1 1215.4 1221.0 1221.4 1232.2 Refractive Index: 1.49840 1.49910 1.49860 SOC (nm/MPa/cm): 32.52 32.61 31.88 31.73 32.06 32.13 CS at 50 μm 980.3 990.1 1006.9 1008.9 991.8 971.9 DOL/410° C./1 mm/ Annealed/Refined (MPa): Time to 50 μm DOL at 17.1 19.3 16.7 17.5 18.0 15.8 410° C./Annealed/Refined (h): Predicted Indentation 19 22 21 27 14 24 Threshold at 410° C./8 h/ Fictivated (kgf): Zircon Breakdown 1220 1250 1215 1218 1231 1223 Temperature (° C.): Zircon Breakdown 32 21 35 37 31 40 Viscosity (kPoise): Example 31 32 33 34 35 Anneal Point (° C.): 630 632 623 634 628 Strain Point (° C.): 576 578 572 581 575 Softening Point (° C.): 903.8 909.1 888.9 909.4 912.8 Density (g/cm³): 2.402 2.4 2.402 2.398 2.399 CTE (×10⁻⁷/° C.): 74.8 75.2 76 74.6 74.2 Poisson's Ratio: 0.224 0.221 0.219 0.218 0.217 Shear Modulus (Mpsi): 4.121 4.118 4.105 4.11 4.118 Young's Modulus 10.086 10.058 10.009 10.011 10.021 (Mpsi): 72-hr Pt Liquidus Temp 990 1005 1000 1010 1005 (° C.): Liquidus Viscosity 980 1000 990 1000 1000 (kPoise): Primary Devit Phase: 2569 2319 1680 1825 1896 200-Poise Temp (° C.): Unknown Unknown Unknown Unknown Unknown 35-kPoise Temp (° C.): 1769.7 1776.1 1756.4 1752.8 1754.2 Fictivation Temp (° C.): 1222.2 1231.5 1208.8 1226.4 1218.8 Refractive Index: SOC (nm/MPa/cm): 32.08 32.11 32.08 32 32.21 CS at 50 μm 970.3 962.8 947.1 940.8 953.0 DOL/410° C./1 mm/ Annealed/Refined (MPa): Time to 50 μm DOL at 16.3 16.3 17.8 17.1 17.0 410° C./Annealed/Refined (h): Predicted Indentation 22 23 25 24 24 Threshold at 410° C./8 h/ Fictivated (kgf): Zircon Breakdown 1219 1232 1226 1236 1227 Temperature (° C.): Zircon Breakdown 37 35 27 31 31 Viscosity (kPoise):

The glasses described herein are resistant to both chipping and scratching, making it well suited for use in cover plates, touch screens, watch crystals, solar concentrators, windows, screens, containers, and other applications that require strong and tough glass with good scratch resistance.

While typical embodiments have been set forth for the purpose of illustration, the foregoing description should not be deemed to be a limitation on the scope of the disclosure or appended claims. Accordingly, various modifications, adaptations, and alternatives may occur to one skilled in the art without departing from the spirit and scope of the present disclosure or appended claims. 

The invention claimed is:
 1. A glass comprising: a. at least about 50 mol % SiO₂; b. at least about 10 mol % R₂O, wherein R₂O comprises Na₂O; c. Al₂O₃, wherein Al₂O₃(mol %)<R₂O(mol %); d. 3-4.5 mol % B₂O₃; and e. at least 0.1 mol % of at least one of MgO and ZnO wherein B₂O₃(mol %)−(R₂O(mol %)−Al₂O₃(mol %))≧3 mol %, and wherein the glass is ion exchangeable.
 2. The glass of claim 1 wherein the glass has a zirconia breakdown temperature that is equal to the temperature at which the glass has a viscosity in a range from about 30 kPoise to about 40 kPoise.
 3. The glass of claim 1, wherein the glass comprises three-fold coordinated boron cations.
 4. The glass of claim 1, wherein the glass is ion exchanged and has a layer under a compressive stress of at least about 600 MPa, the layer extending from a surface of the glass into the glass to a depth of layer of at least about 30 μm.
 5. The glass of claim 4, wherein the compressive stress is at least about 800 MPa.
 6. The glass of claim 1, wherein the glass is ion exchanged and has a Vickers crack initiation threshold in a range from about 20 kgf to about 30 kgf.
 7. The glass of claim 1, wherein the glass is ion exchanged and has a Vickers crack initiation threshold in a range from about 30 kgf to about 35 kaf.
 8. The glass of claim 1, wherein the glass comprises: from about 9 mol % to about 22 mol % Al₂O₃; from about 9 mol % to about 20 mol % Na₂O; from 0 mol % to about 5 mol % K₂O; at least 0.1 mol % MgO+ZnO, wherein 0≦MgO≦6 and 0≦ZnO≦6 mol %; and, optionally, at least one of CaO, BaO, and SrO, wherein 0 mol %≦CaO+SrO+BaO≦2 mol %.
 9. The glass of claim 1, wherein the glass contains less than about one inclusion per kilogram of glass, the inclusion having a diameter of at least 50 μm.
 10. The glass of claim 1, wherein the glass comprises 9 mol % to about 22 mol % Al₂O₃ and from about 9 mol % to about 20 mol % Na₂O.
 11. A glass comprising: a. 66-74 mol % SiO₂; b. at least about 10 mol % R₂O, wherein R₂O comprises 9-20 mol % Na₂O; c. Al₂O₃, wherein Al₂O₃(mol %)<R₂O(mol %); and d. B₂O₃, wherein 4.5 mol %≧B₂O₃(mol %)−(R₂O(mol %)−Al₂O₃(mol %))≧3 mol %, and wherein the glass is ion exchangeable.
 12. The glass of claim 11, wherein the glass has a zirconia breakdown temperature that is equal to the temperature at which the glass has a viscosity in a range from about 30 kPoise to about 40 kPoise.
 13. The glass of claim 11, wherein the glass is ion exchanged and has a layer under a compressive stress of at least about 600 MPa, the layer extending from a surface of the glass into the glass to a depth of layer of at least about 30 μm.
 14. The glass of claim 13, wherein the compressive stress is at least about 800 MPa.
 15. The glass of claim 11, wherein the glass is ion exchanged and has a Vickers crack initiation threshold in a range from about 20 kgf to about 30 kgf.
 16. The glass of claim 11, wherein the glass is ion exchanged and has a Vickers crack initiation threshold in a range from about 30 kgf to about 35 kgf.
 17. The glass of claim 11, wherein the glass comprises at least about 0.1 mol % of at least one of MgO and ZnO.
 18. The glass of claim 11, wherein the glass comprises: from about 9 mol % to about 22 mol % Al₂O₃; from about 3 mol % to about 10 mol % B₂O₃; from 0 mol % to about 5 mol % K₂O; at least 0.1 mol % MgO+ZnO, wherein 0≦MgO≦6 and 0≦ZnO≦6 mol %; and, optionally, at least one of CaO, BaO, and SrO, wherein 0 mol %≦CaO+SrO+BaO≦2 mol %.
 19. The glass of claim 11, wherein the glass contains less than about one inclusion per kilogram of glass, the inclusion having a diameter of at least 50 μm.
 20. The glass of claim 11, wherein the glass comprises 9 mol % to about 22 mol % Al₂O₃.
 21. A glass comprising a. 66-74 mol % SiO₂; b. at least about 10 mol % R₂O, wherein R₂O comprises Na₂O; c. Al₂O₃, wherein Al₂O₃(mol %)<R₂O(mol %); and d. 3-4.5 mol % B₂O₃, wherein B₂O₃(mol %)−(R₂O(mol %)−Al₂O₃(mol %))≧3 mol %, wherein the glass comprises three-fold coordinated boron cations, and the glass is ion exchangeable.
 22. The glass of claim 21, wherein the glass is ion exchanged and has a layer under a compressive stress of at least about 600 MPa, the layer extending from a surface of the glass into the glass to a depth of layer of at least about 30 μm.
 23. The glass of claim 22, wherein the compressive stress is at least about 800 MPa.
 24. The glass of claim 21, wherein the glass comprises at least 0.1 mol % of at least one of MgO and ZnO.
 25. The glass of claim 21, wherein B₂O₃(mol %)−(R₂O(mol %)−Al₂O₃(mol %))≦4.5 mol %.
 26. The glass of claim 21, wherein the glass comprises: from about 9 mol % to about 22 mol % Al₂O₃; from about 9 mol % to about 20 mol % Na₂O; from 0 mol % to about 5 mol % K₂O; at least 0.1 mol % MgO+ZnO, wherein 0≦MgO≦6 and 0≦ZnO≦6; and, optionally, at least one of CaO, BaO, and SrO, wherein 0 mol %≦CaO+SrO+BaO≦2 mol %.
 27. The glass of claim 21, wherein the glass contains less than about one inclusion per kilogram of glass, the inclusion having a diameter of at least 50 μm.
 28. The glass of claim 21, wherein the glass is ion exchanged and has a Vickers crack initiation threshold in a range from about 20 kgf to about 30 kgf.
 29. The glass of claim 21, wherein the glass is ion exchanged and has a Vickers crack initiation threshold in a range from about 30 kgf to about 35 kgf.
 30. The glass of claim 21, wherein the glass comprises 9 mol % to about 22 mol % Al₂O₃ and from about 9 mol % to about 20 mol % Na₂O. 